fix: fall back to asarray when DLPack export of a read-only index fails; buffer error fix

[amalia-k510]

Was running into issues with certain scanpy tests when testing with jax arrays. Example:

FAILED tests/test_pca.py::test_mask_defaults[jax_array-float32] - BufferError: Cannot export readonly array since signalling readonly is unsupported by DLPack (supported by newer DLPack version).
FAILED tests/test_pca.py::test_mask_var_argument_equivalence[float32-jax_array] - BufferError: Cannot export readonly array since signalling readonly is unsupported by DLPack (supported by newer DLPack version).
FAILED tests/test_pca.py::test_mask_defaults[jax_array-float64] - BufferError: Cannot export readonly array since signalling readonly is unsupported by DLPack (supported by newer DLPack version).
FAILED tests/test_pca.py::test_mask_var_argument_equivalence[float64-jax_array] - BufferError: Cannot export readonly array since signalling readonly is unsupported by DLPack (supported by newer DLPack version).

This is the same problem we already handle in __array__ a few lines above, where the comment notes we can't round-trip jax arrays through DLPack and should let numpy handle it in asarray instead.

The fix keeps from_dlpack as the fast path and only falls back to asarray if it actually fails. The array API spec says from_dlpack raises BufferError in exactly this case, so catching it is the intended way to handle it.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.84%. Comparing base (c37e213) to head (f604050).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/anndata/compat/__init__.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2561      +/-   ##
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- Coverage   87.81%   85.84%   -1.97%     
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  Files          49       49              
  Lines        7799     7802       +3     
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- Hits         6849     6698     -151     
- Misses        950     1104     +154     

Files with missing lines	Coverage Δ
src/anndata/compat/__init__.py	85.71% <50.00%> (-4.64%)	⬇️

... and 7 files with indirect coverage changes